Systems have been designed to receive radio frequency (RF) electromagnetic transmissions for a variety of purposes. One application of particular interest involves the receipt of RF transmissions by the system for use in direction finding (DF). In such DF applications, RF transmissions from a signal source are received by the antenna and processed by a receiver and processor for phase and amplitude signature recognition to determine the "direction" of the source.
For ground-based systems, the antenna is typically stationary and the directional information obtained is used to locate the signal source. In airborne-based applications, the antenna is carried by an aircraft and the directional information obtained may be used to locate the aircraft for navigation. In that regard, if the position of the signal source is fixed and known, DF operation of an airborne system can be used to determine the aircraft's position relative to the fixed signal source and, hence, the aircraft's location. Alternatively, if the aircraft's position is known, DF operation of system 12 can be used to locate the signal source.
One example of a system that employs direction finding is the communication intercept (COMINT) system, which receives RF transmissions in the form of communication signals to locate the signals' source. Another example of a system that may employ direction finding is the sonobuoy reference system (SRS), in which the signal sources used for direction finding are sonobuoys used to track submarines.
The most elemental form of DF system determines the azimuth of RF transmissions at the antenna system in a single horizontal plane. In addition to determining this azimuth information, the processing section of the antenna system can be constructed to determine the elevation of the received transmissions in a vertical plane. Range data can also be obtained by further processing of the received RF transmissions.
As previously suggested, systems employed for direction finding can be broadly classed as ground systems or airborne systems. Conventional ground systems employ three vertical dipole antenna elements. The amplitude and phase characteristics of transmissions from a signal source are correlated to stored data obtained from the same elements under known conditions during a calibration procedure. The bearing of a signal source is then determined from the stored data that most nearly correlates to the received data. The construction of such ground systems limits the environmental disruption of received transmissions, generically referred to as perturbation, and, when correlated to stored data, the phase and amplitude of the RF transmissions received by such a ground-based dipole system are relatively accurate.
Conventional airborne DF systems employ a relatively large number of antenna elements that are aligned on two axes of the aircraft and spaced, for example, one-half, three and one-half, five and one-half, nine and one-half, et cetera wavelengths apart. Signals received from relatively widely spaced elements are used to provide accurate phase difference information, while more proximate elements are used to resolve ambiguities.
As will be appreciated, the large number of antenna elements required by such airborne DF systems leaves a limited amount of space for other sensors on the aircraft's exterior. Thus, it would be desirable to employ a system having just a few antenna elements. DF systems employing fewer antenna elements have not been developed, however, because the aircraft's irregular surfaces and peripheral equipment, such as the wings, engines, and other protrusions, would perturb the received RF transmissions, making direction finding inaccurate.
Limited efforts have been made to study the performance of airborne antenna systems. One approach that has been used, however, involves a scaled model of the antenna that receives correspondingly scaled RF transmissions.
More particularly, the spacing of the elements in the antenna to be modeled is typically established as a function of the wavelength or frequency of the RF transmissions to be received. Thus, to allow the use of an antenna model that is physically smaller than the actual antenna, the RF transmissions used during modeling must have a frequency that is proportionally higher than the frequency of the transmissions to be received by the actual antenna. For example, if the model antenna is one-tenth the size of the actual antenna, the frequency of the RF transmissions used to evaluate the model's preformance must be ten times that of the RF transmissions to be received by the actual antenna.
Conventional modeling has, however, been employed as a tool used mainly to design antennas and antenna systems. For example, the model can be used to predict how a cockpit antenna will interfere with a radio antenna, allowing both antennas to be designed and spaced accordingly. Further, prior art antenna models for evaluating blade-type antennas have been primitive monopoles formed by a simple "stub" element, one-quarter wavelength long. These primitive models performed suitably over only a limited frequency band. Also, because such models did not physically model the actual antenna elements, they did not model the perturbations produced by the antenna elements. Thus, prior art arrangements have not been suitable to correct for the environmental perturbations experienced by airborne antenna elements.
In conclusion, it would be desirable to provide a DF system including a relatively small number of antenna elements that can be positioned in an environment in which RF transmission perturbations are likely to occur without such perturbations unduly influencing the DF capability of the system.